An endoluminal device or prosthesis typically includes a stent and a fabric layer, or graft, supported by the stent. A stent is an elongated device which serves to affix the prosthesis in place by providing a radial force against a lumen wall, in addition to supporting the graft. The graft is typically made of a fabric or textile which has a low permeability with respect to the fluid, such as blood, flowing within the prosthesis. The graft may be fully supported by the stent along the entire length of the graft. Alternatively, the graft may have regions which are not directly supported by a stent or stent portion. The graft may be disposed radially outside or inside of the stent.
A prosthesis may be used to treat a vascular aneurysm by removing the pressure on a weakened part of an artery so as to reduce the risk of rupture. Typically, a prosthesis is implanted in a blood vessel at the site of an aneurysm endoluminally (i.e., by so-called “minimally invasive techniques”) in which the prosthesis, restrained in a radially compressed configuration by a sheath or catheter, is delivered by a deployment system or “introducer” to the site where it is required. The introducer may enter the body through the patient's skin, or by a “cut down” technique in which the entry blood vessel is exposed by minor surgical means. When the introducer has been threaded into the body lumen to the prosthesis deployment location, the introducer is manipulated to cause the prosthesis to be ejected from the surrounding sheath or catheter in which it is restrained (or alternatively the surrounding sheath or catheter is retracted from the prosthesis), whereupon the prosthesis expands to a predetermined diameter at the deployment location, and the introducer is withdrawn. Stent expansion may be effected by spring elasticity, by balloon expansion, or by the self-expansion of a thermally or stress-induced return of a memory material to a pre-conditioned expanded configuration. Various types of stent architectures are known in the art, including many designs comprising a filament or number of filaments, such as a wire or wires, wound or braided into a particular configuration.
One common application for the implantation of prostheses is for treatment of abdominal aortic aneurysms (AAA). Such prostheses are typically placed into the aorta and iliac bifurcation with a covering to isolate the aneurysm from the blood. After the aneurysm has been isolated for some time, endoleaks may occur due to worn fabric or other reasons. For example, “type I endoleaks” are leaks occurring at the junction of the lumen wall and the most distal end of the prosthesis (i.e., furthest from the access point). Because the isolated aneurysm may become weak as a result of being isolated, once the leak starts, blood flow and pressure is slowly restored to the aneurysm, and the aneurysm may rupture. Currently, leaks are detected during follow-up angiograms and MRIs, but if the follow up visit does not coincide with the duration of time within which the leak must be treated, the undetected endoleak may result in a ruptured aneurysm that can be fatal to the patient.
Thus, prostheses placed into the vasculature divide the region of placement into a space that provides for the flow of blood and a space where blood flow is excluded. The prosthesis, therefore, is subjected to systolic blood pressure on the side exposed to blood flow (typically the inside surface of a prosthesis) and to some different pressure on the other side. In the treatment of aneurysms by use of a prosthesis, the elimination of growth of the aneurysm sac depends upon the device reducing the pressure on the outside of the prosthesis relative to the pressure on the inside of the prosthesis. It is desirable to monitor this pressure differential to determine if the device has remained effective during its implantation period as a way of checking whether leaks have occurred.
Present attempts to measure pressures within aneurysms require the use of invasive procedures where transducers are introduced via catheters. The pressure measurement is taken relative to atmospheric pressure, so that the precision of the measurement is compromised by the need to operate over a range of absolute pressure from atmospheric up to systole. Thus, there is a need to aid in determining whether any leaks have occurred in a more precise and non-invasive manner.